Colony-stimulating factor 1 (CSF1) is the principal colony-stimulating factor produced by osteoblasts constitutively and in response to several hormones and cytokines. It is absolutely required for osteoclastogenesis, but its role in regulating mature osteoclast function remains unclear. In vitro, it is known to prolong mature osteoclast survival and stimulate cytoskeletal remodeling and motility. We have found that CSF1 can regulate electrolyte transporters and gene transcription in mature osteoclasts, pointing to new metabolic pathways regulated by this cytokine. Recent studies have identified CSF1 as a therapeutic target in states of pathologic bone remodeling such as bone loss associated with inflammatory arthritis and bone destruction associated with skeletal metastases. CSF1 also appears to play an important role in mediating estrogen-deficiency bone loss. Therefore, a better understanding of how CSF1 acts to regulate mature osteoclast function is clearly of physiologic and therapeutic importance. The central hypothesis of this application is that CSF1 mediates mature osteoclast function in part by modulating the activity of the small GTPase, Rac1. We postulate that Rac1 lies at the intersection of two important effector pathways, one that targets actin remodeling and a second that targets gene regulation by modulating NF:?B signaling. Three Specific Aims are designed to test this hypothesis. Specific Aim 1 explores the role of two Rac isoforms, Rac1 and Rac2, in mediating CSF1's actions in osteoclasts because we have defined CSF1-dependent, non- redundant roles for these isoforms in regulating the osteoclast cytoskeleton. Breast Cancer Associated Protein 3, is a Rac-interacting protein we have recently identified in a yeast two hybrid screen using an osteoclast cDNA library. We have discovered that BCA3 regulates CSF1-induced cytoskeletal remodeling and also augments NF:?B p65 signaling. In the second Specific Aim, the role of BCA3 in mediating both CSF1 and RANKL signaling will be explored. Finally, in Specific Aim 3, a genetic approach will be used to further clarify the function CSF1 in mature osteoclasts by deleting its receptor, c-fms, in these cells. In the aggregate, these studies will provide a more detailed metabolic map of CSF1's actions in mature osteoclasts, which is a current gap in the scientific database. This new information will help inform ongoing efforts to develop therapies based on the actions of CSF1 in bone. PUBLIC HEALTH RELEVANCE: Bone loss occurs in a number of diseases such a periodontitis, where bone is lost in the jaw, osteoporosis, where bone loss occurs throughout the skeleton and malignant disease in bone, where bone loss can be generalized or most severe at the site of the metastasis. In all these conditions, the cells that cause the pathologic bone loss are osteoclasts, specialized bone-resorbing cells that move along the surface of bone removing the mineralized tissue. A key regulator of osteoclast function is a molecule called Colony Stimulating Factor 1 (CSF1). CSF1 regulates the formation of new osteoclasts and the activity of mature cells. Drugs and antibodies that block the action of CSF1 are showing early promise as a new way to prevent abnormal bone loss. This application seeks to better understand how CSF1 regulates the function of mature osteoclasts so that new therapies to control the function of these cells can be developed. In particular, we hypothesize that the intracellular protein Rac1, plays a central role in mediating the effects of CSF1. Understanding the contribution Rac1 makes to the metabolic pathways regulated by CSF1 in osteoclasts is the focus of this application.